The Michelin-starred Fat Duck, in Bray, Berkshire, was shut for more than two weeks from 24 February after 400 diners complained of having fallen ill.

The HPA, investigating the outbreak, said the vomiting bug was detected in six staff and eight diners.

It found there were "weaknesses" in how staff illness was dealt with.

Based on staff interviews, sickness records and samples taken, staff worked while still infected with norovirus, the agency stated.

It also revealed the number of cases of reported illness among customers, going back to late January, had now risen from 400 to 529.

Stomach bugs

HPA guidance states that people should not work in food preparation while they have symptoms that are infectious to avoid the risk of passing the infection to others.

Norovirus, known as the "winter vomiting disease", is the most common cause of stomach bugs in the UK.

The virus, which causes sudden vomiting and diarrhoea, is easily transmitted from one person to another by contact with an infected person or through contaminated food or drink or touching contaminated surfaces or objects.

Earlier, Mr Blumenthal, 42, told Australia's Hospitality magazine that the cause of the health scare was "categorically not food poisoning".

The chef said support for the restaurant had been "incredible" but it had been a costly experience for the business.

He added: "For the last five years we've been sending food off every month for sampling and I don't know any other restaurant in the country that does that."

The HPA said the restaurant was safe to re-open for business last week.

But the HPA's Thames Valley Health Protection Unit said: "Results of some tests are still awaited and the questioning of people who reported illness is likely to continue for some weeks."

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